Alkynes and hydrogenation of alkynes

here you will learn more about alkynes and their hydrogenation

1. ALKYNES AND HYDROGENATION OF ALKYNES

2. What are Alkynes? Alkynes are a category of hydrocarbons containing a Carbon-Carbon triple bond. Due to the triple bond , an Alkyne has 4 fewer hydrogen atoms than it corresponding alkane Alkynes form both cyclic and non-cyclic molecular structures .

3. GENERAL MOLECULAR FORMULA OF ALKYNES Acyclic alkynes have the general molecular formula of C n H 2n-2 Alkynes that have triple bonds at the end of the Carbon chain are called monosubstituted alkynes or terminal alkynes which is termed as the most acidic hydro carbon group. Disubstituted alkynes have internal triple bonds

4. ALKYNES PREPARATION The carbon-carbon triple bond of the alkynes is formed the same way as the double bond of the alkenes, by the elimination of atoms or groups from two adjacent carbons. The groups that are eliminated and the reagents used are essentially the same as in the preparation of alkenes.

5. CHEMICAL PROPRTIES OF ALKYNES Molecular structure:- i )The Carbon-Carbon triple bond has the linear geometry. ii) The triply bonded carbon atoms are sp hybridized Reactivity of alkynes:- Alkynes show many reactions with chemical reagents most of them are addition reactions. However, terminal alkynes undergo substitution reactions too.

6. PHYSICAL PROPERTIES OF ALKYNES Alkynes are compounds which have low polarity and have physical properties that are similar to that of alkanes and alkenes. They are insoluble in water. They are quite soluble in the usual organic solvents of low polarity ( eg. Ether, carbon tetrachloride, etc.). They are less dense than water. Their boiling points show an increase with increase in the number of carbon. They generally have a slightly higher boiling points that the corresponding alkanes and alkenes .

7. NOMENCLATURE: IUPAC Find the longest chain containing the triple bond. Change – ane to – yne . Number the chain, starting at the end closest to the triple bond. Give branches or other substituents a number to locate their position.

8. EXAMPLES OF NOMENCLATURE All other functional groups, except ethers and halides have a higher priority than alkynes .

9. BOND LENGTHS Triple bonds are shorter than double or single bonds because of the two overlapping pi orbitals

10. CATALYTIC HYDROGENATION OF ALKYNES Two molecules of hydrogen can add across the triple bond to form the corresponding alkane. A catalyst such as Pd, Pt or Ni needs to be used for the reaction to occur. Under these conditions the alkyne will be completely reduced, the alkene intermediate cannot be isolated.

11. HYDROGENATION WITH LINDLAR’S CATALYST The catalyst used for the hydrogenation reaction is partially deactivated (poisoned), the reaction can be stopped after the addition of only one mole of hydrogen. The catalyst used is commonly known as Lindlar’s catalyst and it is composed of powdered barium sulfate , coated with palladium poisoned with quinoline The reaction produces alkenes with cis stereochemistry

12. MECHANISM OF LINDLAR’S CATALYST Both substrates, the hydrogen and alkyne , have to be absorbed on the catalyst for the reaction to occur Once absorbed the hydrogens add to the same side of the double bond giving the product a cis stereochemistry.